1. Field of the Invention
This invention pertains to improvements in circuits and methods for operating polyphase dc motors, and more particularly, to improvements in methods and circuits for operating sensorless 3-phase dc motors through coil phase commutation techniques.
2. Relevent Background
The present invention pertains to polyphase dc motors, and particularly to brushless, sensorless polyphase dc motors that are used for rotating data media, such as found in computer related applications, including hard disk drives, CD ROM drives, floppy disks, and the like. In such computer applications, three phase brushless, sensorless dc motors are becoming more popular, due to their reliability, low weight, and accuracy.
Motors of this type can typically be thought of as having a centrally located stator with three coils connected in a "Y" configuration, although actually, a larger number of stator coils are usually employed with multiple motor poles. In normal operation, the coils are energized in sequences in which current paths are established through various coil combinations, in, for instance, six commutation sequences. More particularly, generally in the conventional operation of three phase motors, current is passed through two coils of the "Y" connected coil configuration, while the third coil is used to generate back emf (bemf) to be sensed for commutation purposes. The maximum achievable speed is related to the maximum torque that the coil arrangement can physically generate, hence, the maximum current capable of being passed through both coils. This in turn is limited by both the back emf produced by the spinning rotor, and the series resistance of the two commutatively selected coils. All possible current combinations for any two of the three terminals (the center tap being reserved for back emf sensing) yields a total of six equi-distant magnetic vectors to commutate through, spinning the rotor.
At high speeds, the back emf induced by a spinning motor limits the amount of current that can be applied to the coils. This in turn limits the torque, and hence, the maximum achievable speed for a given voltage. The amplitude of the back emf is determined in part by the number of flux linkages or coil turns. Presently, a dual coil mode is used to operate sensorless motors.